The techniques of interconnecting edges of metal sheets, or opposite edges of the same metal sheet, have been the subject of considerable attention, particularly in connection with the construction of large diameter tubular structures wherein an elongated strip of sheet metal is helically wound with its edges interlocked. This technique is used, for example, in making silos. Examples of machines for accomplishing this are found in U.S. Pat. Nos. 3,407,640; 3,838,498; and 3,863,479; in British Pat. No. 899,446; and in German AS 22 10 055.
It has been found that some folded edges of the type shown in the prior art are not stiff enough to connect to edges of, for example, a helically wound strip for a large tubular structure such as a silo, and the danger therefore exists that the folded edge can give way in the axial direction of the tube. In addition, there is a danger that the folded edge can yield and fail when radial forces occur in the fold area. For these reasons, the fold is beaded or rolled so that a standing, double longitudinal fold develops as shown in the above German reference.
Such beading of the fold is possible, however, only when the thickness of the metal sheet is less than about 1/15 of the length of the part extending perpendicular to the metal sheet, or the height of the folded edge. Whenever, the thickness of the sheet is greater, great difficulties are encountered in the continuous formation of the folded edge into a standing, double longitudinal edge. This is particularly true for metal sheets having a thickness greater than 3 mm.